Added a very early version on github. Currently it works by using [mention]USERNAME[/mention] or [mention]USER_ID[/mention]. In the coming weeks I will add a javascript frontend so @USERNAME can be used.

Will there be option to use BBCodes in this way: [mention=USERNAME]user's name[/mention] or [mention=USER_ID]user's name[/mention]?

I'm asking, because in my language we use declension and it really doesn't look good if the name is used in its default form. It can be also useful if user wants to use real name of mentioned person instead of his/her username.